1. Field of the Invention
The field of the invention is optical sensors used in industrial processes and in particular optical sensors having means for aligning their fields-of-view with an area of interest.
2. Background Art
The use of optical sensors is known for the control of industrial processes and for the inspection of manufactured goods. For example, such devices may provide continuous gauging of product, confirmation of package filling, or detection of surface defects.
An optical sensor may be no more than an individual photocell, but for more demanding applications, the optical sensor will be an array of photosensitive elements. The photosensitive elements may be arranged over a rectangular area (a rectangular array) or arranged along a line (a linear array). The rectangular array produces a rectangular matrix of image data suitable for forming a two dimensional image. The linear array may also produce a rectangular matrix of image data by the simple expedient of scanning the array over the imaged object, typically by having the imaged object move past the linear array on a conveyor belt or the like.
Precise alignment of the optical sensor is generally desired to ensure that the full spatial resolution of the photosensitive elements is effectively employed. Ideally, the image projected on the photosensitive elements will span the entire imaging surface of the optical sensor so that no sensor element is unused and no portion of image is missed. A coarse alignment of the optical sensor may be made by sighting along the housing of the sensor or by simple geometric calculations. When precise alignment is required, however, these techniques are not acceptable.
The image produced by the optical sensor might be used to make a precise alignment of the field-of-view of the optical sensor, but an image is not always available. Both the linear and rectangular array may provide sufficient data to produce an image, however, often this data is used without conversion to human viewable form. The data may be processed directly by the process control system or interpreted and compiled for statistical purposes without intervening display. When it is desired that an image be produced, the image displaying terminal is often remote from the optical sensor, either to protect the former from the adverse environment to which the optical sensor is exposed, or for reasons of space or convenience. A remotely produced image is of limited value for alignment. In addition, when a linear array is used, a human viewable image may require that the imaged object be moving, a condition that may not be obtainable during the initial installation and alignment of the optical sensor.
In each of these cases, no human viewable image is available to assist in the alignment of the field-of-view of the optical sensor with the area of interest of the imaged object.
One method of providing for precise alignment of an optical sensor without displaying an image is by projecting a light beam along the field-of-view of the optical sensor so as to create an alignment image that coincides with the field-of-view of the optical sensor. In one implementation, a beam splitter/combiner may be positioned between the photosensitive elements and the imaged object. The alignment image is directed into the beam splitter/combiner to be projected along the axis of the field-of-view of the optical sensor. The alignment of the optical sensor may then be performed by moving the optical sensor so that this projected alignment image is superimposed on the area of interest on the imaged object. If no imaged object is available, a reflecting card may be positioned in the area of interest and the projected image superimposed on that card.
The alignment image may be generated by a specially shaped filament of a bulb which is sized to accurately represent the dimensions of the imaging surface of the photosensitive elements.
One strength of such a system is that the projected alignment image may have the same divergence as the field-of-view of the optical sensor to permit accurate gauging of both the field-of-view's angular position and its size at various distances.
There are, however, two disadvantages to such a system. The first is that the accuracy of the alignment depends on precise positioning of the beam splitter and the projecting source. Any error in the angular position of the beam splitter/combiner will be "amplified" by the length of the optical path as measured between the photosensitive elements and the imaged object. This is true also with errors in the relative angular position of the projection source. Replacement of the projecting source, for example, if it is a bulb, or physical shock, may cause such errors. In either case, if the projected alignment image does not coincide with the field-of-view of the optical sensor, the effectiveness of the alignment image will be reduced.
The second disadvantage to the above described system is that the addition of a beam splitter/combiner and a projection source substantially increases the complexity of the optical path within the optical sensor.